Concept 8 4 Enzymes speed up metabolic reactions

Concept 8. 4: Enzymes speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • An enzyme is a catalytic protein • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -13 Sucrose C 12 H 22 O 11 Glucose C 6 H 12 O 6 Fructose C 6 H 12 O 6

The Activation Energy Barrier • Every chemical reaction between molecules involves bond breaking and bond forming • The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) • Activation energy is often supplied in the form of heat from the surroundings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -14 A B C D Free energy Transition state A B C D EA Reactants A B DG < O C D Products Progress of the reaction

How Enzymes Lower the EA Barrier • Enzymes catalyze reactions by lowering the EA barrier • Enzymes do not affect the change in free-energy (∆G); instead, they hasten reactions that would occur eventually Animation: How Enzymes Work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -15 Free energy Course of reaction without enzyme EA with enzyme is lower Reactants Course of reaction with enzyme DG is unaffected by enzyme Products Progress of the reaction

Substrate Specificity of Enzymes • The reactant that an enzyme acts on is called the enzyme’s substrate • The enzyme binds to its substrate, forming an enzyme-substrate complex • The active site is the region on the enzyme where the substrate binds • Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -16 Substrate Active site Enzyme-substrate complex

Catalysis in the Enzyme’s Active Site • In an enzymatic reaction, the substrate binds to the active site • The active site can lower an EA barrier by – Orienting substrates correctly – Straining substrate bonds – Providing a favorable microenvironment – Covalently bonding to the substrate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -17 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Substrates Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Products are released. Substrates are converted into products. Products Active site (and R groups of its amino acids) can lower EA and speed up a reaction by • acting as a template for substrate orientation, • stressing the substrates and stabilizing the transition state, • providing a favorable microenvironment, • participating directly in the catalytic reaction.

Effects of Local Conditions on Enzyme Activity • An enzyme’s activity can be affected by: – General environmental factors, such as temperature and p. H – Chemicals that specifically influence the enzyme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Effects of Temperature and p. H • Each enzyme has an optimal temperature in which it can function • Each enzyme has an optimal p. H in which it can function Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -18 Optimal temperature for typical human enzyme Rate of reaction Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) 0 40 60 Temperature (°C) 20 80 100 Optimal temperature for two enzymes Optimal p. H for pepsin (stomach enzyme) Rate of reaction Optimal p. H for trypsin (intestinal enzyme) 0 1 2 3 4 5 p. H Optimal p. H for two enzymes 6 7 8 9 10

Cofactors • Cofactors are nonprotein enzyme helpers • Coenzymes are organic cofactors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Enzyme Inhibitors • Competitive inhibitors bind to the active site of an enzyme, competing with the substrate • Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -19 A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme Normal binding A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor Competitive inhibition A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor Noncompetitive inhibition

Concept 8. 5: Regulation of enzyme activity helps control metabolism • Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated • To regulate metabolic pathways, the cell switches on or off the genes that encode specific enzymes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Allosteric Regulation of Enzymes • Allosteric regulation is the term used to describe cases where a protein’s function at one site is affected by binding of a regulatory molecule at another site • Allosteric regulation may either inhibit or stimulate an enzyme’s activity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Allosteric Activation and Inhibition • Most allosterically regulated enzymes are made from polypeptide subunits • Each enzyme has active and inactive forms • The binding of an activator stabilizes the active form of the enzyme • The binding of an inhibitor stabilizes the inactive form of the enzyme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -20 a Allosteric enzyme with four subunits Regulatory site (one of four) Active site (one of four) Activator Active form Oscillation Nonfunctional active site Allosteric activator stabilizes active form. Inactive form Stabilized active form Allosteric inhibitor stabilizes inactive form. Inhibitor Allosteric activators and inhibitors Stabilized inactive form

• Cooperativity is a form of allosteric regulation that can amplify enzyme activity • In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -20 b Binding of one substrate molecule to active site of one subunit locks all subunits in active conformation. Substrate Inactive form Stabilized active form Cooperativity another type of allosteric activation

Feedback Inhibition • In feedback inhibition, the end product of a metabolic pathway shuts down the pathway • Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -21 Initial substrate (threonine) Active site available Isoleucine used up by cell Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Feedback inhibition Enzyme 2 Active site of enzyme 1 can’t bind Intermediate B theonine pathway off Enzyme 3 Isoleucine binds to allosteric site Intermediate C Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine)

Specific Localization of Enzymes Within the Cell • Structures within the cell help bring order to metabolic pathways • Some enzymes act as structural components of membranes • Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 8 -22 Mitochondria, sites of cellular respiration 1 µm